1. Field of the Invention
The invention relates to a method and apparatus for cleaning out particulate matter (dust) that accumulates inside of thermal oxidizers. This invention proposes a means for cleaning the dust out of a thermal oxidizer, without disassembly, during or while the thermal oxidizer is shut down.
2. Description of the Related Art
Thermal oxidizers are used in a variety of systems and processes to decontaminate exhaust and waste gases produced elsewhere in a system or process. Increasingly, thermal oxidizers are being used to treat exhaust gases generated by the thermal treatment of contaminated soil by soil remediation systems. The soil remediation process creates exhaust gases containing much more very fine particulate matter (dust of 150 microns or less in diameter) than previously encountered in other, non-soil, remediation systems that use thermal oxidizers. This application of thermal oxidizers in soil remediation systems requires much more frequent cleaning of dust out from their interiors than previously encountered by other thermal oxidizers used for other processes. Prior to this invention thermal oxidizers had dust cleaned out of them by means that required the unit to be shut down, cooled down (usually taking 4 to 6 days), disassembled, and then manually cleaned out. Another previous practice was the inclusion of separate and additional mechanisms and/or apparatuses to physically remove the dust from the flue gases prior to its entry into the thermal oxidizer.
Krigmont et al. (U.S. Pat. No. 4,987,839) disclose a system for the removal of particulate matter (dust) from combustion flue gases. In practice, systems like this are used to remove dust from the flue gases prior to its entry into a thermal oxidizer for thermal treatment. This system relies upon the addition of a conditioning agent into the gas stream prior to its passing through an electrostatic precipitator. The conditioning agent is used to enhance removal of unwanted dust.
Reducing dust accumulation inside the thermal oxidizer by having dust removed from the flue gases before it enters the thermal oxidizer has the undesirable effect of preventing this dust from being decontaminated by the action of the thermal oxidizer. Consequently this contaminated dust must be further treated and decontaminated before it is disposed of, requiring additional effort and expense.
The addition of a conditioning agent to the flue gases also increases the total amount of particulate matter that the electrostatic precipitator must remove in order to prevent fouling of the thermal oxidizer. This system is an additional and separate device from the thermal oxidizer that has its own effort and expense required to fabricate and operate it. There is also the additional cost of supplying the conditioning agent which is constantly consumed during operation. Electrostatic precipitators also restrict the flow of flue gases through the system. This flow restriction reduces the entire system's throughput and efficiency.
Von Seebach et al. (U.S. Pat. No. 5,365,566) disclose an apparatus for treating the exhaust flow from a modified cement kiln burning hazardous waste. Their system for treating exhaust gases is directed at the problems caused by the formation and accumulation of low-melting point alkali chloride salts. This was done by using a number of cyclonic separators connected to each other by a system of ducts. The scale and complexity of this method limits its cost effectiveness and makes it impractical for use in smaller or portable systems. These same factors also reduce this system's cost effectiveness when it is used in larger stationary installations.
Cyclone separators in general have been observed to have difficulty in removing dust particles smaller than 150 microns in size. This leaves the smaller dust particles suspended in the gas stream until it enters the thermal oxidizer where the gas stream loses velocity as a result of increased cross sectional area. At the lower velocity inside the thermal oxidizer, the gas stream can no longer carry these smaller dust particles along with it. These smaller dust particles then separate out of the gas stream to accumulate on the bottom inside the thermal oxidizer.
When cyclone separators are used in conjunction with thermal oxidizers, there is a pressure drop of 20 to 30 percent across the cyclone separator. This was observed by the inventors when they attempted to solve their dust accumulation problem by using cyclone separators. This pressure drop directly translates into a reduction of flue gas throughput by 20 to 30 percent which degrades system efficiency accordingly. Another problem observed when using cyclone separators is that the dust removed from the flue gases before treatment in the thermal oxidizer has yet to be decontaminated. This dust has its own additional treatment and disposal problems.
Wager et al. (U.S. Pat. No. 5,501,161) disclose a process that uses a filter to remove the dust from a gas stream that is at a sufficiently high temperature to treat certain particulate matter contaminants that are in the dust. This process was never meant to treat and decontaminate flue gases sufficiently for discharge out to the atmosphere nor does it effectively deal with dust removal from a gas treatment device such as a thermal oxidizer. As is the case with prior practice thermal oxidizers, this system must periodically be shut down, allowed to cool down (a process that can take as long as six days), and then disassembled to be cleaned manually. Another problem with this apparatus is the expense of filter elements made to withstand the high temperatures at which this system operates.
Birmingham et al. (U.S. Pat. No. 4,444,735), Bayer et al. (U.S. Pat. No. 5,376,340), and Klobucar (U.S. Pat. No. 5,352,115) all disclose heat exchangers incorporated as an integral part into a thermal oxidizer. Most thermal oxidizers in use today have some kind of heat exchanger or multiple heat exchangers incorporated into the overall system or into the thermal oxidizer itself. Heat exchangers of a variety of types and designs for use with or in thermal oxidizers are commercially available.
Although not directly related to the removal of dust that accumulates inside thermal oxidizers, investors such as Falla (U.S. Pat. No. 1,794,006), Maxwell (U.S. Pat. No. 2,242,653), Reilly (U.S. Pat. No. 2,983,234) disclose a variety of mechanical means to remove ash from various combustion systems. When we tried a number of these mechanical devices to remove dust from a thermal oxidizer, we found them to all have a number of problems in common with each other.
The very hot and very fine dust that accumulates inside a thermal oxidizer tends to stay put unless it is directly pushed. Gravity is insufficient to get the dust to fall by itself into these various mechanical devices. When the dust was directly moved by mechanical means, the heat of the dust warped the moving parts and its abrasive qualities caused rapid wear. This breakage and wear of mechanical dust removal devices built into the thermal oxidizer was a problem directly experienced in our early dust removal experiments.
We also experienced the loss of system efficiency coupled with the problem of contaminated dust disposal that was presented by devices that remove dust prior to treatment of flue gases and the high cost of fabrication and maintenance for all of these systems.